This application is based upon Japanese Patent Application No. Hei. 10-72082 filed on Mar. 20, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to micro-heaters, and particular to a micro-heater having a thin film heater portion being bridged over a hollow portion formed in a substrate, manufacturing the same, and an airflow sensor using the same.
2. Related Art
Recently, an airflow sensor or a humidity sensor formed on a semiconductor chip has been developed. A micro-heater, which is necessary during detection, is provided in this structure. In this case, the micro-heater has a thin film structure of several micrometers in thickness in order to prevent heat from releasing to a substrate during an increase in temperature, and further has a thermal insulating structure such as through holes provided in the thin film structure. In addition, the size of the micro-heater is reduced so that thermal responsiveness and power consumption are reduced.
In general, the thin film structure applied to the micro-heater has a structure in which a heater material is sandwiched by a pair of protection films. Here, the heater material is made of a conductive material such as platinum (Pt), silicon (Si), nickel chrome (NiCr), tantalum nitride (TaN), silicon carbide (SiC), or tungsten (W). The protection film is made of an insulating material thin film such as magnesium oxide (MgO), silicon dioxide (SiO2), silicon nitride (Si3N4), tantalum oxide (Ta2O5), or aluminum oxide (Al2O3).
However, in a conventional film structure, a distribution of internal stress in the structure is generally not adjusted during a room temperature and rising temperature. Therefore, the structure may be deformed to warp in a convex (domed-shape) or a concave (dished-shape) manner due to the internal stress generated in such a way and that remains in the structure. The warpage is further changed due to a difference of thermal expansion coefficients of each material in accordance with temperature changes of the strucuture. As a result, thermal stress is generated in the structure. In this case, a cold cycle is repeatedly substantially applied to the film structure during turning on or turning off of a power supply or during intermittent operation, and it may decrease the reliability of the film structure and the heater portion.
A countermeasure to prevent a decrease in reliability due to the thermal stress has been proposed in JP-A-8-107236. According to this countermeasure, the heater material is sandwiched by an upper film and a lower film having the same material as that of the upper film. The thermal stress is decreased by compensating for the stress generated by expansion and contraction of the upper and the lower heater material by according (equalizing) thermal coefficients of the upper and the lower heater material. It can enhance an effect of decreasing the thermal stress by according thickness of the upper and the lower heater material. The heater material made of platinum (Pt) is set to 0.5 xcexcm, and the thickness of upper film and the lower film both of which are made of tantalum oxide (Ta2O5) are set to 1.25 xcexcm.
However, this structure may also decrease the reliability of the film structure due to the internal stress of the film structure itself, when the thickness of the film is increased for the purpose of increasing mechanical strength. In the case where a compressive stress film is applied, a buckling phenomenon may occur as the internal stress increases. To the contrary, in the case where a tensile stress film is applied, cracks may occur in the film as the internal stress increases.
However, this structure may also decrease the reliability of the film structure due to the internal stress of the film structure itself, when the thickness of the film is increased for the purpose of increasing a mechanical strength. In the case where a compressive stress film is applied, a bucking phenomenon may occur as the internal stress increases. On the contrary, in the case where a tensile stress film is applied, cracks may occur in the film as the internal stress increases.
Furthermore, in the case where the upper film and the lower film are made of Ta2O5 or Al2O3, the upper film and the lower film may be easily changed to poly crystalline film during high temperature thermal process (anneal) of for the purpose of increasing a characteristic of the heater material. Here, since the internal stress of the poly crystalline is different from each other depending on crystal grain sizes, the internal stress may be influenced by the thickness and a manufacturing method, and therefore it is difficult to control the internal stress. Furthermore, since the internal stress may not be uniformly generated in the film itself, it is difficult to control the warpage based on stress previously calculated.
This invention has been conceived in view of the background thus far described and its first object is to reduce a warpage of a film structure to improve durability for thermal stress by structurally adjusting an internal stress generated during fabrication or thermal process.
Its second object is to enable to freely change a thickness of the film structure by adjusting the internal stress.
Its third object is to improve a mechanical strength of the film structure.
Its fourth object is to reduce a warpage of a film structure even when a thickness of the film structure to improve a mechanical strength thereof.
According to the present invention, a micro-heater comprises a thin film heater portion including: a heater layer; a first thin film laminated with one surface side of the heater layer, and formed by laminating a plurality of films including a compressive stress film and a tensile stress film; and a second thin film laminated with another surface side of the heater layer so as to sandwich the heater layer with the lower thin film, and formed by laminating a plurality of films including a compressive stress film and a tensile stress film. Here, the compressive stress film and the tensile stress film of the first thin film and the compressive stress film and the tensile stress film of the second thin film are provided so that an internal stress of the thin film heater portion is released. According to this structure, a warpage stress due to an expansion or compression of the heater layer can be released, and it can prevent reliability of the thin film heater portion from decreasing even when the thickness each film is increased for the purpose of increasing the mechanical strength. Therefore, it can improve durability for a thermal stress with reducing the warpage.
According to another aspect of the present invention, one of the first thin film has an internal stress for warping the thin film heater portion in a domed-shape, and another of the first thin film has an internal stress for warping the thin film heater portion in a dished-shape. Furthermore, each of the first thin film and the second thin film respectively has a film structure so that the internal stress for warping the thin film heater portion in the domed-shape and the internal stress for warping the thin film heater portion in the dished-shape are cancelled each other. According to this structure, an amount of warpage of the heater layer itself due to the internal stress can be reduced when if the internal stress is changed in accordance with temperature changes of the heater layer, because the heater layer is laminated at the substantially center portion of the thin film heater portion. Furthermore, it can prevent the thin film heater portion from warping to one side, because the internal stress for warping the thin film heater portion in a domed-shape and the internal stress for warping the thin film heater portion in a dished-shape will be cancelled.
According to far another aspect of the present invention, the substrate is annealed before forming the heater layer at a temperature equal to or higher than a highest temperature used in another annealing process to be conducted after forming the heater layer; or the substrate is annealed before forming the second layer at a temperature equal to or higher than a highest temperature used in another annealing process to be conducted after forming the second layer. By employing this process, it can prevent an internal stress newly generated at another annealing process before forming a next film from generating.